mod_layout is an Apache module to wrap pages.  This means that you can
add at both the beginning (header) and the end (footer) of the document
some information.  By using the Layout directive, you can actually
dynamically insert layouts anywhere in the document.  This allows you to
create a single look and feel throughout a website without using server
side includes (SSI) to "automagically" add content in either of these
two locations.
